The invention relates to a circuit arrangement for zero point balancing of an integrated operational amplifier having a feedback resistor connected at its inverting input. The circuit has a null balance capacitor which, by means of corresponding actuation of semiconductor switches, is respectively inserted during a balancing phase into a circuit branch lying between the inverting input of the operational amplifier and a circuit point carrying reference potential, and during a following operating phase upon polarization reversal is applied between this reference potential point and the non-inverting input of the operational amplifier.
With a circuit arrangement of this type, it is possible to compensate the offset error of operational amplifiers such that a zero potential is present at the inverting differential amplifier input of the operational amplifier. Nonetheless, an output voltage is emitted by the zero point balancing circuit. This output voltage deviates from zero and its magnitude corresponds to what is referred to as an offset voltage.
With the known circuit arrangement cited above, a null balance capacitor is pre-charged via a feedback branch during a balancing phase to such a degree that, in the following operating phase in which the capacitor is applied to the other amplifier input given a change of polarization, it compensates the offset voltage or the corresponding zero point error.
When semiconductor switches are employed for the switch-over between the balancing phase and the operating phase, then the precision of the compensation achieved in such fashion is diminished due to the influence of parasitic capacitances of at least one of these switches which falsify the charge of the null balance capacitor. An increase in the capacitance of the null balance capacitor to such a degree that this effect is negligible would lead to overall structural sizes which are not practical in the context of integrated circuits.
The described charging of the null balance capacitor usually occurs periodically and during a respective timespan which is short in comparison to the operating time phase. Since the offset voltage is not a constant quantity, the result is that the null balance capacitor may pick up charges under certain conditions which do not correspond to the interference conditions over the entire operating phase. This is the source of a further imprecision of this compensation circuit.
Over and above this, one must also count on interference pick-up at the output of the operational amplifier. This likewise influences the charging of the balance capacitor via the feedback path, and thus also contributes to the fact that the compensation does not have the desired precision.
Disturbances which are coupled in on the grounded line of such a circuit arrangement have a similar effect. During the transition from the balancing phase into the operating phase, a discontinuity of the output voltage of the operational amplifier occurs given the known circuit arrangement. This likewise is undesirable.